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What is "alternate takeoff" in this PT6A performance table?
I am flight mechanic, and I would like to know about this performance limitation:

What is the definition of alternate take off?
Why is the alternate take off temperature limit (ITT) lower than max. cont. ITT?

You need to ask these questions separately

@Abdullah, these 2 questions are inexorably linked.

How should I correct this question ? @MichaelHall

@Richard, I don't think you need to, it is a good question. You may want to clarify whether or not this is for a specific aircraft. I have flown two planes with different versions of the PT6 and I have never taken off at anything other than max power...

MichaelHall Thank you for your answer. I just suspect and need to understand about this subject because every summer the ITT will reach 825 Of Alternate Take off but still not over max. power ITT. So I need any information to clarify our team, sir.

Alternate Take-off is a reduced power option feature, similar to flex thrust on turbofans.  The aircraft manufacturer will provide performance data applicable to the reduced power setting that can be used when circumstances permit.  It's all about reducing wear and tear by not running the engine as hard if it's not essential.
The Max Continuous power setting is intended for emergencies (mostly single engine climb, or both engine climb in some marginal situations, or maintaining altitude single engine where you are one engine and high terrain makes it imperative to not descend) and as such is rarely used, so there is a higher limit to allow even a tired engine to make Max Continuous HP in a critical situation.
The reduced AT ITT limit reflects the objective of limiting wear and tear with a power setting used regularly, whereas the MC limit reflect the balls-to-the-wall limit the engine can take for extended periods when the power is desperately needed.

Note that the alternate take-off ITT limit is lower than for MCT, but the torque limit is higher. Normally you don't hit both limits at once, but one or the other depending on the weather (mainly temperature), so it might make alternate take-off unavailable at high temperatures.

Or on a worn out engine that's running out of ITT margin.

@JohnK Thank you very much, sir.

@JanHudec Do you have information that why Alternate T/O  torque limit is higher than MCT ?

Specifically the Alternate Takeoff is a 10% derate, based on the numbers in the table.


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Can anyone identify this biplane from a TV show?

Could someone help identify this biplane from a TV show, please? No other scene / camera angle is available though.

could you at least name the tv show and episode?

Sorry, it's "Falcon Crest" # 075 <4.07>, but I thought it's no help because this is the only visible angle in a very short segment.

My vote would be a Waco UPF-7 without the engine cowl:

Source: American Power Museum
Looks like the one in the OP post has a different engine in it or at least a relocated cooler, but the struts and windscreen design look very similar. (Struts on the right side are distorted by perspective, look at the ones on the left). Gear bracing is also pretty close.

Looks like a direct hit +1! Struts, gear, windshield all fit. Only the air intake is most likely an oil cooler.

I originally called it a cooler but second guessed myself...

@TJP Please don't add "thank you" as a comment. If you want to thank someone, up-vote the answer. If you think this answer fully answers your question, you can also mark it as accepted (using the tick mark next to it).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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What is this jetliner seen in the Falcon Crest TV series?

I'm trying to identify this jetliner from the "Falcon Crest" TV series filmed in 1984  It's from episode # 068 <3.28> "Ashes to Ashes".  This is the only scene featuring the plane. There are other scenes with planes (stock footage) in this episode, but they feature different jetliners, which have been identified already.  Unfortunately, there's no shot of the cockpit and tail available.  Does anyone recognize this make and model, please?

It is a Grumman Gulfstream II business jet, not an airliner.
The oval windows are very unique to both the Grumman Gulfstream I business turboprop, and Grumman Gulfstream II business jet. The small engines are Rolls Royce "Spey" engines meaning it is either a Grumman Gulfstream II or Gulfstream III. (later Gulfstream versions had the larger "Tay" engines) The lack of wing tip "winglets" means it is has to be a Grumman Gulfstream II.
Grumman Gulfstream II

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Low hour Pilot wants to move up to Piper Malibu Mirage but having trouble finding insurance
I am a 350 hour instrument rated pilot and current. I own and fly a Piper Saratoga II TC and I am rated for complex, high performance, retractable wheel airplanes. I have approximately 150 hours in the PA 32. Current age 59.
I want to move up to a FIKI factory installed airplane and have been mentored that perhaps the PA 46 Piper Malibu Mirage would be a great next step.
Having issues with companies willing to insure me even though my Saratoga has similar systems just not quite as fast. Any suggestions from anyone on what I can do except fly more hours to get insurance?

Have you contacted the aopa?

Welcome to aviation.SE! I'm not sure what you're asking here. Insurance companies set their own requirements to act as PIC if you want their insurance; if you don't like what they're proposing then of course you can negotiate (e.g. agree to fly more hours with a CFI before flying solo) or look for another company (perhaps using a specialized aviation insurance broker).

You won't be able to get around the time-in-type requirement for insurance. That's pretty much a set-in-stone requirement, especially for a high performance aircraft like the Mirage. I suggest that while you build the hours you fly with someone who meets the requirements for insurance.

Thanks. So far I’m being told that I need 750-1000 hours. Seems excessive. I have not contacted AOPA as yet will see if they have some suggestions.

That would be total time, not time in type. I am not surprised. Shop around for other insurance and talk to AOPA, but I don't think the requirement will drop that much.


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How do I find the shear center of an unsymmetrical airfoil?
I'm quite a newbie of these systems and I am designing the wing box of a drone. I know where I want my shear centre. However, I have been trying to determine the shear centre of my idealized airfoil. My airfoil is the NACA6412 and it's highly unsymmetrical.
I know this subject have been touched in this forum before and I have read T.H.G Megson a lot, but I think I am missing something.
I have been stuck in this problem for 3 months now and if someone can provide some light on it, it would be great. My goal is to implement this determination in a script or in excel for future situations.

The shear center depends on the structural construction, e.g. where the spar, shear box, etc. are located.

This is not enough for a response because I don't know the exact method to find the shear center, but I think assuming a constant thickness shell you could come up with a reasonable numeric approximation by dividing the airfoil shell into a number of short straight segments?

This is actually a not bad question, as shear center can change if ailerons or flaps are applied.  The main force on the wing is bending from G forces.  As long as AR is not too high and airspeed not excessive, a good strong box spar structure should be adequate.  Existing designs may save some time.  Reverse engineer them.

Plot the airfoil on a piece of paper, glue that to a piece of cardboard and cut it out. The shear center is where the center of gravity of that cutout airfoil is when balanced horizontally on a pin.
This is a valid method if the wing skin has the same thickness over the full chord. If you intend to use more material in some corner (say, a torsion box forward of the spar), increase the thickness of the cardboard proportionally to the relative shear stiffness of the corresponding section of the airfoil.
If a section does not contribute to shear stiffness (say, an aileron or a flap), leave this section away.
Background: The resistance to torsion of a hollow structural member is proportional to the circumscribed area and the stiffness of the skin of this structural member.

The best way is to assume the airfoil symmetric (since the location of the shear center in the chord is more important) for a preliminary phase; It will help a lot.
Also, don't forget that idealization is a very rough estimate of the stress analysis of a wing structure or fuselage. It helps, however, with initial pre-sizing.


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Can someone tell me what is this on the overhead panel of the Boeing 777?

I found this on the Boeing 777 and it was on the Captain's side behind the speaker.

That is the sensor for the “cockpit door decompression-sensing latching system”.  Most pilots I know are not aware of this system as it is not usually covered during training.
I have seen this system tested by using your mouth to blow a stream of air over the sensor. This creates a brief pressure differential which automatically unlocks the flight deck door.

Very interesting! Does it actually open the entire flight deck door? My FCOM says something about "two hinged decompression panels". It sounds like only these would open.

Yes, it unlocks the door. The decompression panels are separate items.


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Why would the 737 autopilot A disengage without engine-driven hydraulic pump during heavy demand?

Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand on system A, may cause A/P A to disengage.

Just finding it difficult to understand why the A/P would disengage in this case.

If they are hydraulic actuators then the A/P may well disengage when it commands a movement and doesn’t get feedback that it happened.

Doesn't the 737 have a hydraulically powered pitch feel system?

@Jim that would require multiple simultaneous failures, which aren't catered to normally.

@Jim that makes sense, but that means Either A/P would disengage not just A?

Not necessarily. The system I worked on (not commercial jet) had two actuators in series controlled by different systems (perhaps system B here) which means hydraulic pump from system B might still be working allowing A/P B to still send commands to its actuators.

The Boeing 737 has two independent autopilots that use two independent hydraulic servos for controlling the aircraft:

Autopilot Flight Director System (AFDS)
The AFDS is a dual system consisting of two individual flight control computers (FCCs) and a single mode control panel. The two FCCs are identified as A and B. For A/P operation, they send control commands to their respective pitch and roll hydraulic servos, which operate the flight controls through two separate hydraulic systems.

(Boeing 737 NG FCOMv2 4.20.1 - Automatic Flight - System Description, emphasis mine)
You can see the two autopilots A and B connected to hydraulic systems A and B in the hydraulic system overview:

(Boeing 737 NG FCOMv2 13.20.1 - Hydraulics - System Description)
Without the engine-driven hydraulic pump, the respective hydraulic system relies on the AC electrical pump to pressurize the system:

A and B Hydraulic System Pumps
Both A and B hydraulic systems have an engine–driven pump and an AC electric motor–driven pump. [...] An engine–driven hydraulic pump supplies approximately 4 times the fluid volume of the related electric motor–driven hydraulic pump.

(Boeing 737 NG FCOMv2 13.20.2 - Hydraulics - System Description)
Since the engine-driven pump can supply 4 times the fluid volume of the electric pump, the system pressure can drop during times of heavy demand (e.g. gear operation or flap extension/retraction) when only the electric pump is available. The respective autopilot servos can therefore (temporarily) not function as intended and the autopilot will disengage itself.
This could in principle affect both system A and B. However, the PTU (power transfer unit) can use system A hydraulic pressure to assist system B:

Power Transfer Unit
The purpose of the PTU is to supply the additional volume of hydraulic fluid needed to operate the autoslats and leading edge flaps and slats at the normal rate when system B engine–driven hydraulic pump volume is lost. The PTU uses system A pressure to power a hydraulic motor–driven pump, which pressurizes system B hydraulic fluid. The PTU operates automatically when all of the following conditions exist:

system B engine–driven pump hydraulic pressure drops below limits
airborne
flaps are less than 15 but not up.

(Boeing 737 NG FCOMv2 13.20.4 - Hydraulics - System Description)
Since these conditions are met during flap and slat operation (time of heavy demand on system B), the PTU would likely provide enough pressure to keep autopilot B operational after engine-driven pump 2 has failed. Unlike the PTU on the A320, the system does not work in the other direction. Low pressure in system A is not increased by the PTU and therefore autopilot A is at a higher risk of failing during electric hydraulic pump operation.

What a Rube Goldberg design that system is.  Cobbled together by designers who weren't quite comfortable with going all hydraulic. That had to wait for designs like the 767.

Well, 737 is an archaic relic, and recently Boeing has not been very impressive with engineering and manufacturing anyway.

The diagram and the description you quoted seems to suggest that the 737 PTU only feeds the slats/LE flaps, not the whole B system. I would thus naively expect that the PTU would have no effect in terms of compensating for heavy demand like extending the (TE) flaps and keeping the B autopilot operational. Did I misunderstand something?

@TooTea That's correct, but the time of highest demand is when the LE and TE devices are moving at the same time (flap lever positions 1 to 10). Afterwards, only TE devices extend and the PTU is no longer operating (starting at position 15). It would seem that the electric hydraulic pump is capable of providing enough fluid volume for TE flap extension and autopilot B at the same time.


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Does the battery discharge detector have a self test function on the B738?
Does the battery discharge detector on a Boeing 737-800 have a self test function that causes the light (BAT DISCHARGE) to illuminate and extinguish?
I have not found such a thing in the FCOM but been asked by one of my captains.

What self-test functions maintenance is able to use is beyond the scope of what Boeing thinks pilots should know about (lest we try to use them in flight & get ourselves in trouble with things we don't entirely understand).  As far as anything that we observe or can trigger as operators, I'm not aware of any such self-test. That could be something that the ELEC light would indicate, although offhand I don't recall that being one of the things that light monitors.
Obviously the Lights Test switch will show that the bulbs behind the light itself are working, and if you start the APU without ground power, or have the battery as the only source powering the aircraft for a sustained timeframe (i.e. when terminating the aircraft at the end of the day), then you can observe that the light illuminates -- although under certain conditions like APU start it won't trigger the Master Caution, since illumination then is expected.
But as far as a self-test that causes the light to blink on then off, that doesn't sound like anything I've observed.


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Why does stall warning system require 4 minutes of AC power on the 737NG?
Is it related to the Stall Management Yaw Damper (SMYD) computers starting? But the number 1 stall test is working with no need for waiting.
Please provide sources, if available.

I see that in some online forums there is this 4 minutes waiting, but I can't find it in any official document. Where the "number 1 stall test is working with no need for waiting" comes from?
You ask for sources, but you didn't provide the sources of your question :)


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What originally spins the intake fan when the engine is started?
I've been thinking about doing a presentation about how jet engines work. I've watched some videos and everything seems understandable to me, but I have one question. What originally starts spinning the intake fan? The intake fan is powered by the turbines at the back of the engine as far as I understand, so what powers the fan before there's air in the engine to spin the turbines?
Thank you for any help.

I may have misunderstood your question. Can you clarify: are you asking about how the starter motor works (which is what niels answered) or how the fan starts spinning (which is what I answered)?

I’m interested in the entire process of the engine start, which is why both answers are useful to me; the more I know the better.

OK, got it, then I'll leave the answer as it is...

Thank you for the video. It helped me visualize the answer to my question.

In Ye Olde Times you literally spun the propeller by hand.

Most turbofans have a 2 spool design:

The fan and low pressure compressor (LPC) are powered by the low pressure turbine (LPT) and rotate at a speed called N1.
The high pressure compressor (HPC) is powered by the high pressure turbine (HPT) and rotates at a speed called N2.

Only the N2 stage is connected via an accessory gearbox to the starter motor (this starter motor could be powered by different things, see niels nielsen's answer for details). Therefore, the N1 stage will not be moving initially during engine starting. Only the N2 stage starts to accelerate, powered by the starter motor. Note that air can move through the fan and LPT, even when they are not rotating.
The faster N2 rotates, the more air is now pushed through the engine. The air exiting the HPT still has enough speed to accelerate the LPT as it moves towards the exhaust. This is what starts to accelerate the N1 stage. Typically, you'll see N1 starting to rotate a few seconds after N2.
The N1 speed will remain relatively low during the starting process, until fuel is added. With the added energy from the combustion, a lot more pressure is available in the turbines. This will now increase the N1 speed significantly as more energy is extracted in the LPT.
Eventually, the starter motor can be disconnected as the HPT supports the HPC without external help and the engine becomes self sustaining.

There are several ways this is done. You can use an electric motor running off a battery to spin up the jet engine, or you can use a big fan to blow air into the air inlet of the engine, or do the same with compressed air pipes that feed compressed air into a pneumatic motor that is geared to the engine.
The energy source that runs each of these things can be either built into the airplane, or rolled out to the plane on a cart (called a start cart) and plugged into the engine temporarily.
In some cases you can put the starter motor into the start cart and connect it to the jet engine with a rotating shaft to spin up the engine. The SR-71 used a start cart with two Buick Wildcat V-8 engines in it to start its engines.
It is also possible to use a big brick of solid rocket fuel or a really big shotgun-shell-like cartridge in a cannister that feeds the hot gas from the burning fuel straight into the turbine section of the engine to spin it up. This lets you start jet engines in remote places that don't have start carts available.
BTW shotgun or Coffman starters were also used on piston-engine aircraft and farm tractors, where the hot gas was fed to a pneumatic motor with a piston in it that was geared to the crankshaft.

Alright, this definitely helped, I appreciate the help.

Don't forget shotgun cartridges. The English Electric Canberra and Hawker Hunter fired shotgun cartridges to start the turbine spinning. IIRC they had several redundant shots so they have several attempts they can make before the mechanics need to reload the engine.

@slebetman Flight of the Phoenix shows a shotgun start.

@Schwern, i remember that movie, will edit!!!

If I remember my book on Chuck Yeager's life, he apparently was part of the team that figured out that you could "blow start" a jet, blowing air into it via the exhaust of a previously-started aircraft.  I consider it to be one of the more amusing variants to the "blow air into the air inlet of the engine" approaches you mention!

@CortAmmon, on the topic of amusement, US Air Force jargon for oral sex is an "airstart".

"The SR-71 used a start cart with two Buick Wildcat V-8 engines in it to start its engines." Wow!

What originally spins the intake fan [...]?

There is no "intake fan" term used when talking about turbine engines. There's a fan, there's compressor, and there's an intake. But you can't mix those terms: they all mean different things!
A fan is used to blow air that bypasses the core of the engine. Turbofan engines have fans. Turbojet engines don't.
A compressor is used to provide the cooling and working air for the combustors and the turbine.
An intake is generally the system used to route air to the engine. Typically it has no rotating parts, although it may have parts that translate (move back and forth as the flight regime changes).
The compressor in jet engines - whether turbojet or turbofan - is usually axial, and has several stages. In the simplest of the engines, the compressor and the turbine are on a single shaft. This trades off performance for simplicity. More complex engines may have two independent compressor systems, each powered by its own dedicated turbine system. The shafts for those systems are coaxial. The first (low pressure) compressor is powered by the last (low pressure) turbine. The 2nd (high pressure) compressor is powered by the first (high pressure) turbine then.
The fan may be powered by the same turbine as that of the compressor. That would only work for very low bypass ratio turbofans. At higher bypass ratios, the fan is too heavy of a load during the engine startup, and cannot be coupled to the compressor shaft. Instead, the fan then has its own, free power turbine located behind the core of the engine. The word free refers to the fact that this turbine's rotation is completely independent from the rotation of the core of the engine. In many cases you could mechanically brake the free power turbine during engine startup, and then release it later, although it's a rather unnecessary exercise typically.
As for how the engine starts: there is a source of mechanical power that spins up the engine core - it is called a starter. Starters may be electric, pressurized air turbines, hydraulic, or even pyrotechnic (so called cartridge starters where a slow burning rocket fuel is the energy source).
First, the ignition (spark) source is usually turned on, then the starter spins up the core.
Eventually the core is spinning about as fast as the starter can make it go - it's called motoring, and modern engine management systems may display the MOTORING flag in the engine speed readout. The fuel is then turned on to the engine, and it ignites in the combustor.
The core then accelerates. The starter energy source may need to be left on for some time to help the core go up in speed faster. The startup process is, perhaps counterinitutively, a very hard operating regime for the engine. That's because the turbine engines use the air from the compressor to cool the combustors and, most importantly, the turbine blades. When the engine is initially ignited, the compressor is providing well over an order of magnitude smaller mass air flow, and the cooling of the hot section of the engine is marginal. If the engine is not accelerating quickly enough during a start, it may overheat - especially if the engine is fed with too much fuel. This requires immediate shutdown and usually continued motoring to cool down the engine before another start attempt.
As a rough idea to where the energy goes in a jet engine: about 2/3rds of the power available for extraction by the turbine(s) is used to drive the compressor. The remaining 1/3rd is used either as a thrust source via the jet nozzle, or powers the free power turbine that then drives the fan, or a reduction gearbox that drives a propeller, a rotor, an electric generator (typical in peaker electric plants), or a turbopump (typical in natural gas applications).
Now you can get a hint as to why the engines not get spinning faster during startup before introducing fuel, to reduce the turbine temperature during startup. It takes an enormous amount of power to get the compressor operating at any significant fraction of the idle speed where turbine cooling is sufficient. This speed would be at least ground idle, but sometimes higher than that, up to flight idle speed. To keep starter size and power demands manageable, the starters bring the engine up to a speed that only just allows startup without overheating and not much more than that.
Another reason for low startup speeds is the airflow limit for reliable fuel ignition. Once the fuel is burning, the engine can be brought up to full air flow. But when the airflow is too high, it's almost impossible to ignite the fuel, so there's an upper engine speed limit for reliable ignition and development of stable flame in the combustor.

"At higher bypass ratios, the fan [...] cannot be coupled to the compressor shaft. Instead, the fan then has its own, free power turbine [...]" -> this is true for Rolls-Royce Trent engines with a 3 spool design, but other high-bypass turbofans have the fan connected to the compressor shaft (LPC and LPT shaft), sometimes with a gearbox in between (e.g. PW1000G).


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How does a tailplane provide downforce if it has the same AoA as the main wing?
If an airplane is traveling through a uniform environment and its main wing is parallel to its horizontal stabilizer, how does its tailplane generate downforce? Only explanation I can think of is because it is submerged in the downwash of the main wing as shown in this picture

But if this is the case how is it possible for aircraft which use outboard horizontal stabilizers which extend out of the downwash to be stable?

https://aviation.stackexchange.com/questions/36934/did-wwii-fighters-have-inverted-airfoils-for-horizontal-stabilizers

Do you know for a fact that any designs w/ outboard stabilizers as you've illustrated, have zero difference in incidence between stabilizers and wings?

Don’t forget that they have elevators.

For a stable configuration the incidence of the wing is normally higher than that of the tail. Also, the wing has more camber, so its zero-lift angle of attack is lower. Even at the same incidence the wing therefore will create more lift per area than the tail, the basic condition for static longitudinal stability.
Flying in the downwash of the wing actually has advantages:

increasing the angle of attack also increases the downwash, so the airflow over the tail will see less variation in angle of attack. Once the wing comes close to stall, the tail is still comfortably in the linear range.
Download on the tail in combination with the downwash means that a tail might even produce a bit of thrust.

How does the wing has more lift per area when it has the same aoa as the tailplane, because of the downwash? if they're parallel and also same aoa then OHS config has some downwash on the tailplane too (maybe air out of the wingspan is affected too?) edit: thanks for the useful links.

@Kozakov Please do not confuse incidence (angle versus a reference line defined by the airplane's structure) with AoA (angle versus direction of flow). At the same incidence, both wing camber and downwash will make sure that the tail has less lift per area than the wing. At the same AOA, lift per area will also be the same (actually still a bit less on the tail due to its lower aspect ratio, but that influence should be small).

@Peter, Just a quibble, but to be accurate, won't the airfoil shape, (as well as aspect ratio),  affect the lift per unit area? At the same AOA, a highly cambered wing will generate more lift than a symmetrical wing of the same area. I mean, camber shifts the C$\sf{_{L}}$ curve to the left. Unless you redefine AOA, a cambered airfoil generates lift at zero AOA, a Symmetrical airfoil does not.

@CharlesBretana Stability depends on the lift derivative over AoA, and camber has no influence here. Not the geometrical AoA is important but the difference between geometrical AoA and zero-lift AoA. Camber only shifts the zero-lift AoA to more negative values but leaves lift curve slope unaffected. Camber does, however, make a wing more unstable because of the shift of the center of pressure with angle of attack.

@Peter, Yes, I agree, but you said in prior comment, "At the same AOA, lift per area will also be the same."

@CharlesBretana I was sloppy. What was meant to be the same was the angle between zero-lift AoA and actual AoA. Geometrical AoA is an arbitrary thing, just like incidence. The angle from zero-lift AoA is what counts.

As I said, a quibble! On another note, your comment about downwash creating Lift is interesting, I have a Long EZ, and the winglets on Rutan's designs generate a small amount of thrust from a similar mechanism. The vortices from the lift on each wing cause the airflow impacting the winglet (extending above the wing) to be directed at an outward angle, (away from the fuselage), causing the same effect as downwash on the tailplane of a conventional aircraft.

@CharlesBretana Correct, and that is why some people call them tipsails, in analogy to nautical sails.

tipSails! Yes, that's an excellent...  metaphor?

Many tailplanes are inverted airfoils, such that their "lift-vector" is pointed downwards.
tailplanes also frequently have a negative AoA (eg, aimed slightly downward a few degrees).
The downward force is created by both of these: the upside-down airfoil, and the downward AoA.

I did some more research myself and wanted to post what I have found.
Short answer: Static longitudinal stability does not require download on the tail.
After reading quiet flyer's comment, I  wasn't sure if there was zero difference in incidence between stabilizers and wings of a plane which uses outboard horizontal stabilizers and the drawings in the paper which I've found the ohs image I added to the question looked like they had parallel flight surfaces but I wasn't sure if they were original drawings which they made the tests with.
So I designed and built this small guy which has parallel flight surfaces to be sure.
I started testing by placing CoG somewhere around here.

which caused it to immediately pitch down after hand launching it
I removed weight from the tip until none was left and CoG was shifted all the way to the back of the trailing edge like this, and finally it flew very good at this CoG location.

After searching for "center of gravity" in the paper which I took the OHS image in the question from I found this;

After some more research I came across this which answers my question.

Static stability doesn't req downforce since the tail is just contributing to the overall weathervane effect where the horizontal aerodynamic center wants to trail the C of G in the airstream. Downforce is req'd to TRIM the body to weathervane at a specific angle to the flow away from zero lift.  If the angle of attack is to be positive (nose up) 5 deg, the tailplane must be configured to create static equilibrium at that angle, by creating a 'force fight" of opposing pitching moments that achieves balance at the desired +5 deg off the flow.For that case, the tail must make downforce for trim.

To add to @John's comment, for aircraft without a tailplane (delta wings) the distribution of lift on the wing creates the same effect. We use the concept of a Center of Pressure, or Aerodynamic Center, but in fact, that is an aggregate simplification that is incorrect when analyzing stability. The large upwards lift generated by the forward part of a delta wing is closer to the CoG, and exerts a nose down pitching moment that is balanced by the smaller downwards lift at the trailing edge (From elevons),, which is further from the CoG,

How does a tailplane provide downforce if it has the same AoA as the main wing?

Simply by deflecting the elevator. For horizontal stabilisers that can be trimmed: by adjusting the angle of incidence, and then it won't have the same AoA as the wing.


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Calculating angle of attack during bank
I'm programming a flight dynamics simulation and am running into issues with calculating the angle of attack during banked flight. I know what I'm doing wrong, but I'm not sure what I should be doing instead.
When an aircraft banks, angle of attack and lift are expected to increase in order to compensate for the reduced vertical component of lift: How does the angle of attack vary in turns?
However, in my simulation, angle of attack and lift decrease as the aircraft banks. This is because I am calculating angle of attack using the $u$ and $w$ values in the body frame. As the aircraft banks, the vertical component of gravity in the body frame is reduced, but the vertical component of lift in the body frame stays the same:

This means that it is generating more lift than gravity, causing the simulation to reduce angle of attack to compensate, and it stabilizes at a lower angle of attack. This causes issues like reducing drag, causing the aircraft to speed up as it banks, when I would expect it to slow down.
Assuming the $x$ axis in the body frame is parallel to relative wind, angle of attack should be calculated using the vertical velocity in the body frame. But banking reduces vertical acceleration in the body frame due to gravity, causing lift to reduce as well.
These are the (simplified) calculations I use to get the vertical acceleration components:
$\alpha = \arctan(w / u)$
$L_z = -2\pi \cdot \alpha \cdot q$
$g_z = 9.81 \cdot \cos(\text{bank}) \cdot \cos(\text{pitch})$
$w_z· = L_z + g_z$
So I rotate the gravity acceleration according to bank, but not the lift acceleration. This is what causes vertical acceleration in the body frame to decrease with bank. I've tried flipping it and rotating the vertical lift component instead, but this causes issues with other forces still being calculated in the body frame, and the simulation becomes unstable.
My question is: how do I calculate vertical acceleration in the body frame in a way that acceleration due to lift does not stabilize according to only the vertical component of gravity, but the entire gravity force? Or, how can I get angle of attack to increase in a bank, not decrease?

Does this answer your question?

"Assuming the x axis in the body frame is parallel to relative wind..."?? This angle - between the body x axis and relative wind - is the angle of attack (plus sideslip, if you consider it), by definition.

Your question would be greatly improved if you could clarify what you are trying to model.  Is there a human pilot or autopilot "in the loop", making pitch inputs as needed to achieve some given goal in the turn (such as maintaining constant altitude?)  Or are you trying to model what the aircraft will do "on its own" if the elevator position is left unchanged as the aircraft enters a turn?  There is in fact no tendency for the angle-of-attack to automatically increase as an aircraft enters a turn.  Related:  https://aviation.stackexchange.com/a/76823/34686

It's an axes definition issue, it is important to realise that altitude and weight are earth axis variables.

Centripetal force is provided by lift, not by gravity. Downward gravity force does not reduce when the aeroplane takes on a different attitude, it always pulls downward to earth with magnitude $m*g$.
in the statement "..angle of attack should be calculated using the vertical velocity in the body frame..", "vertical" should be defined in earth axes frame. In a bank angle, gravity is tilted relative to the body frame, however it will cause the aircraft to be accelerated in the gravity vector direction if the compensating force is not equal in opposite direction. "Downward" is always in the direction of gravity.
Best to compute this issue in earth axes via an axes transformation matrix, and then there are many answers on this site on how forces, AoA and altitude should be accounted for. Lift must be larger than weight in a bank, the pilot must increase AoA until the altitude remains constant.

It seems you forgot to add centrifugal acceleration. Lift must increase to produce the centripetal force that is needed for turning. I suggest you start with the desired turn rate and determine the amount of sideways lift from there. This in relation to the amount of lift for compensating gravitational weight will give you the bank angle.
Now it is helpful to split forces into their horizontal and vertical components.
Your equation for $L_z$ gives actually the full lift. You need to split this into its components, namely for becoming $L_z$ it needs to be reduced by the cosine of the bank angle.
Your $g_z$ would become zero without bank. It is always helpful to see what the expressions are for zero and 90° bank. Gravity works only in the vertical plane, so you need no modification for bank angle.
Write down the sum of all forces in both planes and adjust angle of attack such that they balance. You will see that the angle of attack will increase in a turn.

I think where I'm confused is that $w$ is vertical velocity in the body frame, since it's vertical velocity relative to realtive wind. So I'm not sure how to account for the change in horizontal velocity as the aircraft banks - I have the horizontal component of gravity but it does not factor in to AoA calculations.
I have tried balancing the forces, but I run into issues with stablizing them after changes to bank, so it's the dynamic aspect that I have wrong. Per "Your $g_z$ would become zero without bank", I had the wrong equation. I've swapped sin(bank) for cos(bank) in the question.

It appears that you are assuming that there is a human pilot (or autopilot) in the loop, making pitch inputs as needed to prevent the aircraft from accelerating up or down as the bank angle is changed.
(You could also model what the aircraft tended to do on its own if the aircraft entered a bank but the elevator position remained constant.  This would be a completely different--and much more complex-- problem than the one you appear to be describing.  See these related ASE answers (1, 2) for more on this.  There is in fact no tendency for the angle-of-attack to automatically increase as an aircraft enters a turn.  Rather, in the absence of any corrective action on the pilot's part, the flight path tends to curve downward, leading to an increase in airspeed.  The aircraft ends up following a descending helical path, at a higher airspeed than it started with.  To a first approximation, angle-of-attack remains constant, but see the above link for why this is not exactly true.)
You've implicitly set up a scenario where thrust is varied as needed to equal drag, and you want to vary the angle-of-attack as needed to prevent the flight path from curving upward or downward as the aircraft enters a turn.
Your mistake is in assuming that the criteria for the flight path remaining horizontal rather than curving upward or downward is that lift should be equal to weight * cosine (bank angle).  The correct criteria is that lift * cosine (bank angle) should be equal to weight.
If I've misread your question, and your intent really is to describe how the aircraft will react on its own to entering a bank, in the absence of corrective input from a human pilot or autopilot, then your present approach to the problem has numerous other issues that need to be fixed.  You would need to start by re-examining your conception of how angle-of-attack is actually governed during flight.  What you would need to do, for a good first approximation, is hold the angle-of-attack constant, and model how much the airspeed needs to increase in order for the vertical component of lift plus the vertical component of (drag minus thrust) to equal weight, as the aircraft settles into a descending turn.  For shallow descent angles, you can ignore the contribution from the vertical component of (drag minus thrust) and still be "in the ballpark", so that's a good starting point for figuring out how much the airspeed will increase when the aircraft is banked.  The steeper the bank angle (and thus the steeper the descent angle), the more this simpler method will overestimate the actual increase in airspeed.


text:
Where does FlightAware obtain the filed flight plans under the "Flight Data" section?
Visit the FlightAware page for a random flight, say https://www.flightaware.com/live/flight/QFA570
Now under the "Flight Data" section, sometimes we'll see an entry called "Route" with the flight's filed routing.
I've noticed that for flights operating to/from or passing through Australia and New Zealand, the full routing is revealed regardless of direction of flight.
This is in contrast to the situation for flights passing through the US, Canada and Mexico, where flights leaving these places will have the full flight routing available for viewing, but those heading to the US, Canada and Mexico will only reveal the sections of the routing inside their airspace.
i.e. the displayed route on FlightAware for a flight from LHR to JFK's route will start in US/Canadian airspace, whereas a flight from JFK to LHR will have the full route revealed, down to the STAR in London. To illustrate, have a look at these 2 FlightAware pages. BA117 operates from LHR to JFK, and BA116 the other way round.
https://www.flightaware.com/live/flight/BAW117
https://www.flightaware.com/live/flight/BAW116
Now there are some exceptions, notably https://www.flightaware.com/live/flight/VIR3, but the rule generally holds true.
For flights overflying these areas (e.g. from HAV to FRA), the sector which will have the flight first pass through US/Canadian/Mexican airspace will have the full routing revealed. So the full routing is revealed from HAV to FRA, but not from FRA to HAV, in similar fashion to the LHR to JFK situation.
For all other routes which don't fall into the abovementioned category, there is no "Route" entry.
I'm interested to know - where specifically is FlightAware getting this filed route data from? Can I access it and view it independently? Are there sources where I can get this information for flights outside the abovementioned regions?
I am particularly interested in routes in Europe and Asia, especially longer ones, as for shorter routes flights normally take standard routes which you can deduce with the high-altitude airways overlay in FlightRadar24 (if you have a better way of doing so, please let me know). Longer flights however will have their routings change pretty much every flight.
I've tried going into the EuroControl portal, checking various ADS-B websites, but to no avail. Initially I thought https://globe.adsbexchange.com/ might have the routing since it carries so much information but unfortunately not.
Thank you in advance for any help.

What a great question

One of the data sources used by FlightAware is ASDI. This data includes filed flight plans for airborne flights and also (near) future planned flights.
Although the Wikipedia page limits its description to ASDI as provided by the FAA, other ASNPs provide similar data using the ASDI protocols. Generally this data is available by specific arrangement with ANSPs and is not a publically accessible feed. I know that FlightAware does have specific arrangements with ANSPs other than the FAA to obtain this information.

Thank you for the information my friend. Will be reading up on this and seeing what I may potentially stumble upon.


text:
How are shear winds studied experimentally?
I've played extensively with wind shear Math models, but I wonder how the aerospace researchers study it experimentally:  Do they generate shear flows in a water tunnel?  Or do they use a wind tunnel?  Is generating a shear flow experimentally even possible?  I've checked and noticed that there's little-to-no literature for it.
Thanks,

Sometimes they can go in situ.

It may be off topic here, but there were more or less routine experimental measurements taken by NASA.
Wind profiles were measured for the Space Shuttle on the day of launch by releasing "Jimspheres", two meter radar reflective helium balloons covered with stabilizing cones.

The balloons were tracked by the same C-band radars used to track the shuttle stack during ascent.
The system had an altitude resolution of 100 feet, allowing it to pick up wind shears if they were greater than that.
The paper
Space Shuttle Day-of-Launch Trajectory Design Operations contains a full description and is the source of the image.


text:
With existing tech and knowledge of meteorology, can aircraft be designed to reliably improve their range or loiter using updrafts to gain altitude?
Just what the question says. I want to know if aircrafts' range, loiter time, or other measures of merit can be dramatically and reliably increased using our current understanding of meteorology and aerospace engineering to make fixed-wing aircraft that use ambient meteorological conditions to remain aloft, using onboard propulsion as little as practical. If so, what realistic applications do you envision? If you know of any academic literature on this topic, please share it.
I know that recreational glider (i.e., "sailplane") pilots often use thermal columns and some other phenomena to extend their flights. I also know the Perlan project uses rather unique conditions of Patagonia to achieve impressive altitudes. I'm more interested in broad commercial, academic, and military applications: probably usable in a large number of places, autonomous or remotely piloted, and optimized for mass production.
To give you some cues, the broadest application occurring to me is atmospheric satellites for telecom, survey, navigation, etc. More specific applications would be something like loitering munitions or intelligence-surveillance-reconnaissance drones. Other ideas and their explanations are welcome.
Could one achieve loiter times of weeks or months, or would too much reliance on ambient meteorological conditions render an aircraft optimized for "soaring" unreliable?
Neither engineering nor meteorology is my trade, so patient explanations are appreciated. Thank you in advance for your answer.

Gliders don't just "extend" their flights using thermals, their entire flight (except takeoff) is entirely based on thermals

Boeing SolarEagle was a project competing for the DARPA Vulture initiative, consisting in maintaining during 5 years an aircraft of 500 kg aloft at an altitude of 20/30 km , delivering 5 kW of energy to its payload. While this project was cancelled (in favor of a satellite solution), other similar projects may exist.

Thanks for the reference @mins.

@60levelchange I'm unsure there's a meaningful difference between what you and I wrote unless I'm missing your point. Using an external power source to launch or tow a glider into the air "extends" the glider's flight time from 0 minutes to a number greater than 0 minutes. The flight entirely depends on that external power source, yet all it's doing is increasing the flight time beyond 0 minutes.

Thermals are finite. This means they have a useful diameter which is smaller than the diameter of the smallest circle most airplanes can fly. The wing loading of a heavy (= full ballast tanks) glider is around 50 kg/m² and is already too high for thermalling competitively on cloudless days with weak thermals.
Compare that to at least 200 kg/m² for anything commercially useful (e.g. Beech King Air B200 or Do-228 with both at 205 kg/m² when at MTOW). Even the Britten Norman Islander at only 100 kg/m² could only fly around regular thermals, not continuously in them. Given an L/D of maybe 15 for light utility airplanes with retractable gear and 12 or less for fixed-gear airplanes, their minimum sink speed with a wing loading of  100 kg/m² is in the order of 2 to 2.5 m/s. In order to gain altitude, the strength of the thermal at a radius of 80 to 100 m from its core must be at least 3 m/s in order to gain altitude (slowly). Still, they would have to circle most of the time and have preciously little time left to cover some distance if their destination is not directly downwind on a regular day in Europe, or need low wind speeds in order to stay in one place.
In order to carry a meaningful payload, the unmanned airplane with a very low wing loading must be large. Since the inner wing flies a much smaller radius than the outer wing, circles of large airplanes cannot become smaller than several multitudes of their wingspan. Even a very lightweight (and fragile!) airplane could at most have 30 m wingspan and weigh at most 800 kg in order to thermal successfully.
Thermals are only possible when the atmospheric lapse rate near the ground is instable. This condition is only met on maybe 20 or 25% of the days in a year and only between noon and several hours before sunset. And between ground and 3000 m on an exceptional day at moderate latitudes. Even 2000 m are already considered good and only possible towards the end of a day. Staying airborne overnight is impossible, except if a mountain and a steady wind are at hand to stay in ridge lift until the thermals are again strong enough on the next day.
In desert areas like inner Australia, Namibia or the US Southwest, stronger and more reliable thermals are possible. Maybe you can thermal with a large and heavy airplane there, but certainly not in Canada or most of Europe. And the density of customers for the services of atmospheric satellites is rather low in desert areas ...
Atmospheric satellites need altitude in order to cover large areas. With only 2000 m everything above a radius of 150 km will already be beyond the horizon when assuming a grazing angle. This also means that many receivers will be behind the shadow of buildings or mountains, so the useful radius shrinks to 23 km with an angle of 5° to the ground. You will need many airplanes to cover a larger area, and doing so reliability will never be possible.
What is still possible is the use of ridge lift, or to fly straight below lined-up clouds on a windy day with thermals. This can help to reduce power  or fly faster, and clever pilots do this already if they can.

OK, I know this is a silly idea really, but you could shrink the wingspan while keeping the wing loading low - use a bi- or tri-plane.  Some of the downsides of multiplanes don't really apply here as high speed is undesirable.  After all the concept is akin to an (untethered) kite, and kites have been proposed for similar applications

@ChrisH Yes, but. Double deckers or triplanes come with a hefty increase in zero-lift drag, but will be much lighter than a cantilever-winged monoplane. This cuts the need for span and will not affect performance much at low speed, but the plane will be blown away once wind speed picks up.

What kind of "thermodynamic limit" is those  0.65°C per 100 m supposed to be? It is a statically stable stratification, if anything. The adiabatic lapse rate existing in the mixed layer part of the CBL is ~9.8K.km^-1.

Ok, atmospheric satellites are out. Let's say you wanted something less ambitious like a small drone to patrol a 50km stretch of the Indo-Chinese border in the Himalayas. In your opinion (I realize this involves lots of assumptions), what's the maximum payload you could reliably keep aloft with a combination of ridge lift, mountain waves, and thermals? What design features and resulting performance characteristics would the drone need to do this? Will the drone likely need to stray from its 50km patrol zone or points of immediate interest within that zone to follow favorable weather?

0.65°C/100m is simply the average value between the ground an the tropopause. 0.98°C/100m is the cooling rate of rising dry air. The actual conditions vary wildly from one day to another. If one does not take into account humidity - which is a huge factor in reality - then the equation GroundTemp - 0.98 * ConvectionHeight = ConvectionTopTemp holds true.

@VladimirFГероямслава This sentence was stupid. I have edited it.

@Sorghum Reliably? Zero. With a good chance to be useful over some days? Maybe a couple of hundred grams (wild guess). With a good chance to be useful for some hours? Several tens of kilos. The smaller and lighter your drone, the better are its chances to stay aloft, but also the lower will be its speed. And speed is necessary to prevail against strong winds.

It is still quite incorrect. Thermal convection exists in normal latitudes almost every day. Often even in winter. It even exists in places like Dome C in Antarctica, during the "day time", albeit very shallow, a few hundreds of meters and with the thermals being accordingly weak in vertical speed.

@VladimirFГероямслава So thermals occur in stable air? But only when moist air has a lower density than the surrounding dry air, and the resulting vertical movement is normally too small to be useful as a thermal. Maybe you propose a better sentence and I will edit it in.

@PeterKämpf No, they happen in unstable air and continue rising through well-mixed air until stable air stops them. But the unstable air is much more common than you claim it is. It is true that useful vertical velocities only happen in summer  when the boundary layer is deep and the surface heat flux is strong. The vertical velocities of thermals scale as (z_i * g * w'T' / T)**(1/3) where w'T' is the surface kinematic heat flux and z_i is the boundary layer height  https://glossary.ametsoc.org/wiki/Deardorff_velocity

The moisture is actually not too important unless clouds develop, but then in shallow convection the cumulus clouds do not modify the underlying mixed layer that much, they just let the thermals rise higher inside the clouds. In the expression above the moisture flux does appear in the v subscript, but it is usually a small contribution.

@VladimirFГероямслава Thank you for the link! Moisture reduces the density of air compared to dry air of the same temperature. That is why you can find weak updrafts over lakes. Once moisture condensates and contributes its energy to the surrounding air, thermals become stronger but that happens only in clouds. An unmanned airplane might use that updraft, but optical or infrared sensors will be blind when flying in clouds. Practically useful thermals really are rare.

Thank you @PeterKämpf. This has been immensely helpful.

You should not forget the infrared mongolfiers. This system was invented by French Centre National d'Etudes Spatiales (national spatial studies center) in 1977. It can fly between 18000 and 28000 meters for a few weeks with a useful load of 45 kg (ref in french).

Here is an English document (credits should go to @mins)

@mins: I have updated my answer with your reference...

Worry not, I've not forgotten about balloons. I only limited my question to fixed-wing aircraft because I was specifically curious about fixed-wing aircraft. Nonetheless, aerostatic lift seems quite advantageous for increasing duration and payload. Thank you for providing the literature reference.

Yes it is certainly possible to extend flight times almost indefinitely by using atmospheric lift, but there is a trade off between duration and other factors such as a flight plan and movement of payload.  If you’re solely interested in keeping an aircraft aloft with minimal fuel then this is achievable but if you need to get payload to a destination in a set time then it’s typically better to straight-line with minimal consideration of environmental effects.

Last sentence... it doesn't read good. You should write "considering economic factors". We do not choose the faster way, or faster speed, but the most effective considering fuel consumption (cost), crew time (cost), opportunity costs (more flights), fees, etc.

“Doesn’t read well”, perhaps.  I agree that your point is a valid one but what I wrote is what I intended,  i.e. trading off flight time vs fuel as distinct from minimising the overall cost of the flight.

authors: Vladimir F Героям слава


text:
Why do digital flight instruments seem to fluctuate/flicker during landing or takeoff?
I happened to watched videos where landing was aborted due to some or other reasons.
However, something interesting stuck me eyes: the digital flight instruments fluctuate/flicker (such as the attitude indicator) and can be easily noticed apart from the usual flight routine.
Why is it so?
source link 1:

source link 2:

Welcome to Aviation.SE! Could you please clarify if you mean fluctuations of the attitude or speed/altitude values on the screen or the screen itself flickering.

This is something you'll see on live TV if there are LED taillights on cars - the flicker of the LED doesn't line up exactly with the frame rate of the camera and they'll flicker. I don't recall ever having noticed it on a pre-recorded TV show and believe that they may fix it in post-production so they're not flickering.

Posting an example of one of the videos would go a long way towards eliminating speculation.

In short:
The reason you see this is two-fold:

Usual CRT flicker by principle, CRT frames are not displayed in their entirety at any time, but this is not visible to the naked eye, due to the frequency this phenomenon occurs.

A camera is perturbed by any flicker effect, due to the sampling principle it uses. Aliasing takes place, artifacts are created, and are visible.

It may be surprising, but this effect is the result of not fulfilling the first principle of sampling, known as the Nyquist criterion, sampling must done at a frequency at least twice the largest frequency contained in the signal to sample. This can be illustrated like this:

In this image the green signal is sampled at black points. The sampling frequency is obviously lower than twice the signal frequency. When reconstructing a signal from the samples, the result (red) is a different signal. The analogy with CRT is the invisible higher frequency flicker is sampled by the camera at a too low frequency and the result is a low frequency flicker which is visible.

Details
This is an artifact known as stroboscopic effect. It appears when the real world is sampled (temporal sampling) and we look at the succession of samples. Instead of viewing the continuously changing real world we see a sequence of fixed states. This has two effects:

A fixed state may show things we couldn't see before they were frozen, like a picture which is incomplete at a given time.

The sequence of these fixed states may create artifacts, like adding or removing displacements such as a wagon wheel rotating in reverse direction, or a helicopter hovering with stationary blades.

Stroboscopic effect is one of the many form of aliasing. Aliasing refers to an artifact created when rebuilding an object from samples that not in adequate amount to get a faithful copy. In technical term this means the original object was sub-sampled given it's complexity. This relates in fine to the Nyquist criteria: If a signal contains frequencies up to f, then it mustn't be sampled at a frequency lower than 2*f.
In the effect you see there are two reasons: The way a CRT shows images (actually bands on the screen) and the way the camera captures (samples) images. The effect is the combination of the two.
Image creation on the CRT/LCD screen
In a CRT screen, the image is built by energizing phosphor material on the screen surface, using a small electronic beam, line by line from left to right and top to bottom. As phosphor luminosity cannot be maintained a long time, the screen must be continuously scanned by the beam at some frequency, usually 50 or 60 Hz, but today it's often at a higher frequency.
At a given time, the most bright lines of the screen are the one currently refreshed, and a few lines which were refreshed immediately before, which number depends on the persistence of the phosphor used. This creates a brighter band visible when frozen by photography.

Source
Passive LCD screens are similar: A passive LCD cell (pixel) is made transparent by storing electrons in a tiny capacitor which is unable to keep it's charge a long time, so this capacitor must be reloaded periodically. This is also done line by line, and also create areas of different brightness at a given time.
A human eye also shows a persistence of vision. When our eye sees a bright point, the stimuli is maintained after the point has disappeared. This is why we can see a photographic flash which actually lasts less than a millisecond. For most people, there is no flickering after 40 Hz. The refresh rate of the CRT was chosen in order to take advantage of this persistence. The electricity network frequency (50/60 Hz) was used as this provided a stable clocking reference.
Does that mean we need to send 50 (60) images to the CRT each second? Actually this would have been too much for the radio transmission associated with TV broadcast, and only 25 (30) images were sent each second, first all odd lines, then all even lines. The CRT actually displays the two interleaved halves of an image each 1/25 s. This apparent frequency of 50 Hz is enough to prevent flickering for humans.
This is similar to movies which are recorded at a rate of 24 images per second by the camera and are actually projected twice or more this frequency, using a shutter, giving a rate of 48 shorter images per second or more.
Stroboscopic effect from the camera
So a flickering actually happens on any CRT, but we can't see it with naked eyes. However, a video camera actually samples the reality to create, say 25 fixed images per second.
But what do we see on these images: Bands! The location of the bands on the fixed images varies from one image to the next, like in any stroboscopic sampling:

Sampling a moving object at fixed intervals, source
This is the main reason for flickering screen on video recordings: The illuminated portion of the screen varies during the recording, it seems the screen is flickering (while it's actually never totally black).
Now imagine a case where the camera scans exactly the same number of images than the CRT, but is not in-phase, that is the camera doesn't scan the line being refreshed by the CRT, and it happens the camera scans a line where phosphors have already lost their luminosity:

In such a configuration when the camera scans the next line, another line has returned to black, maintaining the same black gap between camera line and first illuminated line on the CRT. In such a configuration the camera would never be able to record a line, creating black images. As the camera and the CRT are not synchronized this happens only from time to time. In between partial images of some random sort are recorded.
This effect can exist unless the refresh frequency of the CRT is high enough compared to the phosphor persistence, or the phosphor persistence is high compared to the exposure time of the camera (if that were achieved, other problems would appear though).
There are ways to remove partially this effect, like temporal anti-aliasing which consists in blending several images together. This way bands at different positions appear on the same image, though it's difficult to equalize luminosity over the synthetic image created.
Other stroboscopic effects due to a camera
As the camera also scans the sensor matrix receiving the image line by line, lines recorded actually belong to different instants. Lines can't match if the objet moved during the assembly of image lines. Because of this a fast moving propeller can appear completely bent:

Video image made of lines belonging to different times, changing the shape of the blades, source
This image is not a high speed photo freezing the propeller, on such photo the propeller wouldn't be distorted. The distortion is caused by the way the camera scans its electronic sensor, line by line, without freezing the sensor content first.
This is an interesting video of similar effects, like these guitar strings:

Source

Omg the picture of the prop  an excellent answer in it's completeness.

Re: the guitar strings. That image does not (as is sometimes claimed) show the strings vibrating at an audio frequency; it is (as you say) only an example of the stroboscopic effect.

@Jpe61 its an "Icarus" branded prop - and flew too close to the sun  (humour)

Note that the interlacing applies to television broadcast, but (these days) not computer screens. Computers generally can send 60 full images a second to the monitor. The stroboscopic effect is, of course, there whether the image is interlaced or not.

Note that the questions specificially asks "during landing or takeoff", which I don't see addressed in this answer. I assume it's is actually just a perception bias, and not specific to landing or takeoff.

Never heard this called "aliasing" before - I've only ever heard that used about interpolated pixels. On the other hand, this IS similar if not the same as a "Moire Effect"

@MikeBrockington: Aliasing is a very generic term used when the sampling frequency is too low for capturing the reality, and leads to seeing details which never existed.

Great answer, though since this is apparently not in any way related to Aviation, rather just being a general issue with screen and camera refresh/sample rates, would it make sense to migrate this question elsewhere?  Like Physics, Electrical Engineering, or Photography maybe?

Would be nice to have a summary of the answer at the top. Something like They're not actually visibly flickering to the naked eye, it's an artefact of the interaction between the displays and the camera

@Muzer: That's a good suggestion, thanks. Done.

You did not mention the type of instrument or aircraft type, but I assume you mean modern cockpit displays.
The flickering you see in the videos is due to refresh rate of the display being off-sync with the camera recording the video.
More over, older CRT displays used for cockpit instruments are updated in two steps: the first sweep draws the lines and the second one draws the colored areas, like the artificial horizon.
The effect is very pronounced in this video:

Comments are not for extended discussion; this conversation has been moved to chat.

Modern cameras automatically increase or reduce frame exposure duration based on available light in order to control exposure.  The exposure duration along with the CRT being drawn quickly is causing this effect.  You can see in this video @ 1:42, the screens are relatively steady, but when the camera view moves to include the windows and the camera adjusts to the increased brightness, the flickering becomes more pronounced @ 2:00.  The OP's first video does the same when the pilot's bright white shirt causes the camera to adjust @ 0:46.

Another possibility - though shouldn't happen in a modern electrical system:
Does it happen once at exactly the same time the throttles are opened for a go-around?  Could be a minor power fluctuation as the engines increase thrust, adding to the current in the 28V circuits.
Suppression and filtering should catch all of that in a modern aircraft, but CRT-based displays can always glitch out briefly for many reasons like voltage/current spikes that would not affect a modern switchmode PSU.
Additionally CRT displays can be tweaked by a stray magnetic field - it doesn't take much to throw a beam of electrons off target, which is perceived as a brief visual flicker.

"it doesn't take much to throw a beam of electrons off target". Actually it's more difficult than you suggest. In a color tube, 3 beams are converged/focused to tiny holes of the shadow mask (or trinitron grid) over each phosphor triad. These beams have to travel various distances depending on the hole location and beam color, but still have to converge at any time, this requires incredibly stable electronic corrections. Such stability makes them largely insensitive to external power source and magnetism transient events.

@mins fair enough.   My experience is of a cellphone operating, or an unshielded speaker very near would cause an effect on a CRT which would be ineffective on an LCD display.

The very powerful unshielded speaker is indeed an extreme, but still it won't make the image flickers, just distort it, and the usual joke is to put the magnet on the screen, that is close to the shadow mask to make it locally permanently magnetized. Expensive joke.


text:
Location on the wing where obstacles least disturb airflow
I have a model airplane whose wings I would like to fit with light strips top and bottom. These light strips would be across a significant portion of the wing's span. Without extensive study (wind tunnel, etc.) is it possible to say where I should put these light strips chord-wise so as to minimally impact lift, stability, etc? Here's an example of such a lit-up airplane:

In this case, it appears the lights are placed by the manufacturer around the half chord point, but I don't know if that's more the exception or the rule.
BTW, I'm planning on trying to bury the strips in the wing, not just sit them on top. But I may not do a perfect job, so I want to have minimal impact.

Excellent video. Mount the lights inside the wing and cover with clear Monokote.

@RobertDiGiovanni That was basically my thought. Of course, the top and bottom sides want to be visually different so I can't take that too far.

Generally, the farther aft a small obstacle is, the less turbulence and drag it makes, because farther aft the boundary layer is more likely to be already turbulent.
However, if the obstacle is heavy, placing it aft might require extra nose weight to restore the location of the CG.  That extra weight might be undesirable.
Also, disturbing the wing's upper surface is more likely to encourage a separation bubble and stall, than disturbing the lower surface (where positive pressure will reattach the airflow, albeit with some drag).  Of course, this doesn't matter for an aerobat that often flies inverted.
Practically, for this airspeed and Reynolds number and power-to-weight ratio, it doesn't matter.  You could even mount the LED strip right on the leading edge, with a ghastly mess of tape and hot-melt glue, and claim that you've added a turbulator strip to make the stall break more manageable.

I love the visuals on this answer.

Positive pressure gradient.

authors: Vladimir F Героям слава


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Are Final Tests always required for LRUs?
Is an aviation component workshop allowed to release a line-replaceable unit (LRU) without going through a Final Test, if the LRU passes the pre-testing procedure and is found to be a No Fault unit?

Welcome to aviation.SE! Because you want to know if something is "allowed" or not, it would be best to say which country you're in and which aviation regulations apply to the aircraft (FAA? EASA?).

As Pondlife said, the question cannot be answered and should be closed as too broad unless you specify the jurisdiction which you are asking about.  Different countries have different rules and the answer will change depending on where you are asking about.  Please update your question and add a tag for the appropriate regulatory authority which you want to know about (faa-regulations, etc.)

I'm assuming by Pre-Testing procedure you are referring to a physical inspection and downloading the fault history.
If there are no logged faults in the fault history, it means that the built-in test (BIT) of the LRU did not detect any faults. BIT is not capable of 100% test coverage. It is also possible that BIT results are inaccurate. A lack of recorded faults is not sufficient to declare a unit airworthy.
Once an LRU is tagged and sent to the service center/shop, it can only be returned to service (RTS) after it has passed the RTS tests specified in the component maintenance manual. After passing the RTS tests, a 'maintenance release document' can be prepared and the unit returned to the spares pool.
In the US, the maintenance release document may be an FAA Form 8130-3, Form 337, or other document containing the information required by 14 CFR Part 43, sections 43.9 and 43.11. The maintenance release document must be signed by an authorized person who is certificated under 14 CFR Part 65. Though not legally required many shops will affix a "yellow tag" to the unit to indicate it's serviceable status.
EASA has similar rules, the primary difference being the required content of the documentation.

What is "yellow tagged"?  Are you assuming a particular jurisdiction?

@Simon  Thanks for the reminder.  Edited answer to better describe documentation and jurisdiction.

Hi Sir lets say that the Pre-Test is the same as the Final test. Our process is something like this "If the unit fails the pre-test , it goes on the trouble shooting to figure out the defective part then repair then final test (RTS) then certification.. But if the unit passes the PRE-TEST , the unit goes directly to certification process." This is what I want to know if  there is any reference documentation that an LRU should go through a Final Test.

@Boy-Z If your 'Pre-Test' meets the requirements for RTS and the LRU passes, it can be released to service. Any LRU that is 'approved' - that is it is built under a TSO or a PMA must have documentation that provides "Instructions for Continued AIrworthiness" or ICA.  In many case this will be in the form of a Component Maintenance Manual (CMM). This ICA or CMM will specify the return to service operating limits that must be met. A test must be performed to verify that it operates within these limits. It doesn't matter what you call the test as long as you document compliance.

to add on to the above answer (since i do not have enough points to comment yet), usually after installing the LRU on board the aircraft, there will be BIT and operational/functional tests to be done to confirm the unit is working.

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Will helicopter control inputs be reversed because of precession if the fuselage starts a rapid spin?
My question stems from a loss of tail rotor effectiveness. If a helicopter with a counterclockwise rotating blade like the R22 loses the tail rotor and the fuselage starts a rapid right turn and thus gains momentum, will the control inputs be reversed or will it create a double gyro from the main rotor system  and cause control inputs to be normal?

It's a good question,, voting to leave open.

Which "control inputs" do you think will be reversed?  This question is confusing.  Are you assuming that the pilot does not reduce collective pitch to reduce the rotation after the tail rotor is lost?

You are correct I am assuming collective stays up, throttle stays on and hover is maintained

@Wes That's counter-intuitive, from a pilot's point of view.

Short answer: no.
What are your flight control inputs?

Collective Pitch.
Cyclic adjustments to pitch and roll
Yaw / anti torque from the pedals, which control the tail rotor.

Let's take them in order.

Collective.  If the control inputs were reversed for collective pitch, you'd not be able to get the torque off by lowering collective (which is the usual response to a loss of tail rotor and the attendant upset it causes by loss of the anti torque function of the tail rotor being lost).  If the helicopter design reverses what the collective does, in terms of pitch increase or decrease, dependent upon the state of your tail rotor, nobody in their right mind will fly it.
Once again for emphasis: if you lose the tail rotor, you reduce collective pitch to reduce the torque, and thus the requirement for anti-torque (which is why the tail rotor is there in the first place).  Check the flight manual for your model of helicopter for further details on that in-flight malfunction / emergency.
Cyclic.  The control linkage between the cyclic and the means of tilting the rotor do not change if your tail rotor fails.  They apply their changes to the "tilt of the disc" as long as they are connected when you move the cyclic.
What happens as the fuselage beings to rotate (particularly if the rate of rotation increases) is that the pilot is at risk of being disoriented by the rotation and making control inputs that make the situation worse.  (See above: first, reduce pitch, and find a place to put the aircraft down per the flight manual).  The rotation of the fuselage does not change control linkages, it changes the OODA loop for the pilot:  Orient/Observe/Decide/Act is a continuous process (loop) when flying such that you make a control input to make the aircraft do something based on your perceiving a need to change.
The problem with a control input made when you involved with the uncommanded rotation of the fuselage is that the pilot's aim point keeps changing.  That isn't a problem of precession, that's a problem of "getting behind the aircraft."  It's a cognitive, not a mechanical, problem to overcome so that this doesn't end in tears.
Yaw Pedals.  Per your problem statement, they don't work, so reversing control inputs isn't an issue.  Your feet are along for the ride.  Your brain and your hands get you to the ground safely, or not.  The way to deal with yaw is via the torque reaction to main rotor pitch: with power on and no tail rotor, changing collective pitch will tend to make the nose move one way or the other.  (Your flight condition: In cruise flight, in a hover, or in one of the many between states that a helicopter can be in will influence that).

The problem with the uncommanded rotation of the fuselage isn't some change in precession, it's the disorientation of the pilot whose reference line keeps changing as the helicopter rotates.  The whole point of being the pilot is to control the aircraft.  The pilot's objective in this case is to reduce the disorienting rotation by reducing collective pitch and flying the aircraft (lacking your anti-torque inputs) as you normally would.  You won't get increased precession (vis a vis the cyclic inputs) due to the anti torque control being lost.  Your rotor system has enough inertia if you keep it at flight rpm where at least that isn't your problem.  Your problem is in lag, and making control inputs that are out of synch with the (constantly changing) orientation of the helicopter.
Depending upon aircraft type and the situation, you will likely need to secure the engine(s) before final landing so that you don't create a rotating situation right before touchdown if you need to add collective while you still don't have the anti torque from your tail rotor.
Check your model's flight manual for the detailed procedures to handle a loss of tail rotor.

II totally understand that we do in fact need to take torque out of the main rotor in order to slow the uncontrolled right turn via hover auto. My question is that after a 180 degree spin from loss of tail rotor is rolling off throttle even enough to stop the spin? Also once you get spinning that fast gyroscopic precession won't come into play on the fuselage? Thanks again for you answer!!!

Wes, how fast do you presume that the fuselage is spinning? Its mass, weight, and inertia profile is quite different than the blades, as is the drag.

@Wes are you asking only about specific case of this happening in a hover?  If that was your question, it needs to be pulled out of the comment there and put into the question. If you are interested in the general case, then OK, maybe that bit about the hover is a follow up question?

Sorry for the lack of information but I was indeed referring to a hover situation

Wes, if you're in a hover and you lose tail rotor drive (rather than stuck pedals, but instead the tail rotor blades not providing any lift/thrus), the last thing you need to be worrying about is precession.  Your flight controls still work as designed, your aim is to get the bird down on the ground safely, and most manuals are going to tell you to reduce torque to slow/stop the rotation as a part of that procedure.  Your most pressing concern with your cyclic control is to try and keep the aircraft as level as possible to avoid dynamic tip over when one of the skids contacts the ground.

That would be ideal if you could catch it as soon as it happened. However you will atlease spin 180 degrees by the time you even recognized tail rotor failure

Wes, if you are in a hover and the nose begins to move in an uncommanded fashion you don't wait to swap ends before you start to deal with this loss of control.  But inertia being what it is, you may indeed swap ends before you put it on the ground, reaction times and surprise considered.  You may find this video of interest (Jefferson City Missouri, Navy Seahawk, 2011).

Very interesting video! Thank you so much for the insight!!

@Wes This post and this thread may be of interest to you It's about some myths regarding rotary wing flying.

Very useful site! Thank you for the recommendation!

Read over your original post and it finally started to sink in! I completely understand  what you mean by the fact that gyroscopic precession wouldn’t change since cyclic controls wouldn’t change (i.e. the control linkage) if the fuselage started to spin! Thanks again for the insight!

@wes glad to be of some help. :)

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Are hot-air balloons regulated similar to jet aircraft?
Can conventional hot air balloons be considered aircraft as far as regulations are concerned? I mean, they do fly in the air, can ascend into the sky and can land, and also can navigate. But it doesn't have wings.
I want to know if the safety regulations on hot air balloons are similar or equal to normal jet aircraft. Is there any restrictions on how hot air balloons should be maintained and piloted? Are they similar to those restrictions imposed on jet aircraft?

It depends on what you really mean. Airspace is regulated (aviation), but for (wild) birds and weather ;-) It doesn't depend on the type of aircraft. For construction, certification, operation there may be different/simplified rules (light aviation, shutes/para, gliders, etc.), but the general principles are the same.

slightly related: What would be the process to approve a minimal, standardized flight recorder for commercial balloons?

Not all aircraft have wings. Helicopters have rotors, dirigibles have helium, some craft have rockets, lifting bodies just omit the wings.
Large balloons are regulated, similar to other aircraft. Here is FAA's list of balloon regulations for the US: https://www.faa.gov/aircraft/air_cert/design_approvals/balloons/balloons_regs


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Won't sonic booms prevent Space X's BFR intercity transport plan from being acceptable?
Considering that Space X have just announced that their BFR can take anyone anywhere in the world in under 1 hour, that would be enormous speed, does this rocket not produce sonic boom?  If not is it because of its shape and that it does not have wings? Will it ever be allowed to fly across the US above the oceans and anywhere at those speeds considering again sonic boom?

hundreds of satellites cross the oceans and continents at this moment, at enormous speeds. Yet nobody complains about sonic booms.

Why worry about sonic booms?  Consider the launch noise.  Then figure that when you launch from anywhere the noise would be acceptable (KFC or Vandenberg, in the US), it would likely take you as long or longer to get there and board the rocket, than it would to go directly to your destination.

@jamesqf I don't know about you, but I can get to KSC much faster than I can get to, say, Hong Kong. And, like most of the world's population, I can get to KFC much faster than to either of those locations. :)

Please tell me this stands for Big F***** Rocket.  (reference: http://doom.wikia.com/wiki/BFG9000)

@PeterCordes Technically it stands for Big Falcon Rocket, but yes, it was deliberately chosen for the similar sound. https://en.wikipedia.org/wiki/BFR_(rocket)

@reirab: Maybe if you live in say Miami :-)  But for anyone east of the Rockies, say, you'd basically have to either drive from your home, or get to an airport, spend hours going through boarding & security, fly to an airport near KSC, deplane and pick up luggage, travel to KFC, spend hours going through boarding & security there...  It's the same problem as with the Concorde: decreasing actual flying time doesn't shave all that much off total trip time.

@jamesqf ...it does if you save 16-28 hours and a stopover?

@jamesqf I live in TN. I could drive 11 hours to KSC and still get to Hong Kong faster with the rocket than the normal way. Last time I went there, it was well more than 24 hours from when I left home to arrival in HKG (and that was with no delays.) 19 hours of actual flight time, plus an hour drive to the airport, plus a layover, check-in, etc. I can get to KFC in about 10 minutes, though. They have pretty good chicken. The Concorde was an alternative to a 6-hour flight. This is talking about an alternative to 15+ hour flights, where the flying time actually is the majority of the trip.

@toonarmycaptain: But you could save a comparable amount of time by taking a direct flight instead.  Do you really think this rocket transportation system will go direct from where you are to where you want to go?  Especially when carrying a large number of passengers, which I understand is the plan.  How many people want to travel from say NYC to Hong Kong at any given time?  Not many, so you have to wait around to collect enough passengers to pay the costs.  The economics are like those of a private jet vs scheduled airline, but upped by several orders of magnitude.

@reirab: Maybe YOU could do that trip in 11 hours, given a big enough gas tank and lots of coffee.  I think for most people it would be a two-day trip.  Google says about 14 hours drive time (not including fuel & rest stops), so not that far off the HK flight time.

@jamesqf There's daily or more flights/connections between all of the routes Musk showed in his graphic. 5 direct flights for NY to Hong Kong, not including connecting routes eg Emirates and other airlines that don't do direct. For 95% of the trips I take, what he's suggesting would be an order of magnitude quicker than presently available. Any A380 flight is a 500 odd passenger capacity,  and often a 8+ hour flight,  so most of those routes would be competitive.

@jamesqf Google says it's a bit over 10 hours from where I live to KSC. Judging from lots of personal experience driving from where I live to central Florida (most recently yesterday,) that's about right.

@toonarmycaptain: How many people on those daily flights are paying for a first-class ticket?  And how many of that small fraction are going to be able and willing to pay the orders of magnitude higher cost of a rocket flight? Then again, there's the issue of not being able to launch from anywhere near NYC or many if not all of those other places, due to noise and safety concerns.

You're assuming (and it is a big assumption), that noise/safety concerns might not be mitigated. Now you're talking about cost? IFF Musk is correct, cost will be competitive. So will time, even if you're 1 hour hyperlooping to a lauch/landing site at each end. But since you asked for numbers, you're talking around 90 between business and economy class for your average Emirates A380. More than enough to fold into 1 or 2 rocket flights per day.

By the way, lived near Aldershot MOD/RAF/whatever they call it now, for a few years...the sonic booms weren't that much of an irritant. Have you ever heard a large jet or a fighter taking off...not exactly quiet. Not on the order of a rocket launch, but they're working on bringing engine/sonic boom noise down, and there's no foreseeable reason why similar research cannot mitigate rocket noise. Some kind of interference/damping, channelling into absorbent materials below the launchsite etc are all valid ideas.

The only way to get anything that far that quickly is to send it into space, and that's exactly what Musk is suggesting. The BFR will launch a passenger carrying spacecraft out of the atmosphere and into a sub-orbital path, it will re-enter the atmosphere close to its destination. Rockets launch pretty much straight up and a sonic boom would have to compete with the rocket itself on noise. Once above the atmosphere there is no sonic boom because there is no air.
On re-entry all spacecraft create a sonic boom, for most of the re-entry this is  inaudible from the ground because the air is extremely thin, once in the lower atmosphere it gets louder. By the time the spacecraft gets low enough to create a really big sonic boom it's usually at a subsonic speed and would not make any boom at all. The shuttle and Apollo spacecraft both created sonic booms on re-entry, that's covered in this Space SE Question.
So, are sonic booms a problem in this idea? It depends, the space shuttle re-entered over the continental US many times and it never bothered anyone, but if it was happening a few times a day it might be a problem. You could get around that for the most part by re-entering over sparsely populated areas or the ocean.
More likely it's not the sonic booms that people won't want from a noise perspective, but the launches.

Since the rocket will be traveling through space, there will be no noise on the ground during cruise.
As pointed by @DeltaLima in the comments, a lot of satellites are crossing the sky above us at this very moment. Since there is no air in space, no sound can be heard, especially from that distance.
The only noisy parts will be the takeoff and landing, but with carefully selected location, this shouldn't be a problem.

Landing presumably won't be supersonic, so booms shouldn't be an issue there. 'Takeoff' (i.e. launch) will be noisy regardless of the boom.

@reirab landing is subsonic, but part of approach is supersonic.

@ChrisStratton True, but, if it's anything like the Space Shuttle, that part of the approach will be too high to matter. Not sure about SpaceX's rocket, but the Space Shuttle descended extremely quickly. It was like piloting a brick.

"a carefully selected location" for takeoff would have to be far from a densely populated area. Which pretty much defeats the point of "getting quickly from A to B". You would first have to get from your area of population to A which is "far from civilization". Seems like a pretty deep flaw in the plan. I hope they have thought this through.

I doubt it. Sonic boom/s may well occur but likely (or could be specifically planned by routing etc) to occur in upper atmosphere/away from population centers.
The rocket seems planned to take off from an island or floating structure (judging by the graphics, which showed a ferry to the launch site), which could easily be tens of miles away from land if that were an issue. That distance might be enough to mitigate a lot of the launch noise.
As for landing, it's probably possible to come into landing along such a trajectory as to not be near a city when excessive noise will be produced, either by coming in over the ocean (in the graphic's example) or even by 's'turning around populated areas, shedding speed in the same trajectory (the space shuttle bled off speed this way, I believe).
Note that military craft produce booms frequently (although AFAIK they try to limit proximity to populated areas), as does any orbital (and most suborbital, although not so many of those fly these days) rocket, and Concorde produced booms. So:

Will it ever be allowed to fly across the US above the oceans and
  anywhere at those speeds

I don't see potential booms impacting whether it is allowed at all. Where it is permitted to be routed/produce booms, and how frequently, are open questions.

The US banned commercial supersonic aircraft flying across land in the US specifically because of the Concorde, so in theory any supersonic "aircraft" produced by Space X flying above the US would also be prohibited if not by the same rule, by the same concerns. However it's a rocket and not an aircraft and so doesn't make the same sonic booms. Perhaps more importantly this shouldn't also be an issue because it's made by a US company, not a foreign one, and Boeing has no plans to make it's own competing version.

Rules can be changed. NASA has been working on technologies to minimise sonic boom related disturbance as part of its research into hypersonic aircraft, so there's some mitigation possibilities there too.

Yes, I explained why I think the rules would be changed if necessary.

@RossRidge Those rules only apply if you're flying in U.S. national airspace. Rockets don't do that, except during launch. Completely different sets of rules involved.

@reirab Yes, as I said "it's a rocket and not an aircraft".

Taking off from an island or floating platform doesn't do a whole lot to reduce journey times unless you live on the coast...

...a lot of the time if you're flying long distance, you already travel to a coastal city for your connection, right?


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does the compressor section of a jet engine creates a depression in front of it, due to air be sucked at a great rate
for large Jumbos like the Airbus A380 and the Boeing 747 - 800 at maximum thrust at take off, does the compressor section of a jet engine creates a depression in front of it, due to air be sucked at a great rate.  Is the air sucked in by the jet engine replaced by the artmosphere as fast as it is sucked.  How does this compare with the aircraft at cruise speed and altitude and what impact does it have on aerodynamics and air resistance relative to speed with which it is replaced and the speed of the aircraft also considering the large area around the aircrat with which theair is sucked

Your question would be better received if you ask "does the compressor section of a jet engine creates a depression in front of it, due to air be sucked at a great rate."

@ymb1: This answer provides some information, though not much, and some uncertainty remains about the lower pressure extent .

I tried to answer as far as I did understand the question. Could you please rephrase your question with respect to the section about friction. I am not sure what you are asking here.

If the air wasn't replaced you'd hav a vacuum... Also you really need to improve on the wording of your questions. If you need help, ask in chat. Finally, there's no such thing as a Boeing 747-800, it's a 747-8, see here

(I am not entirely sure if I understood the question, but I hope you are asking how the air-flow into the jet-engine changes between stand-still and flight.)
On the ground (stand-still and taxi) the intake of a jet-engine will "suck" in the air. This means the pressure in the inlet section is lower than the ambient pressure.
In-flight the engine is "rammed" into the air. Here the pressure in the inlet-section is higher than the ambient pressure (see sketch from Wikipedia).

+1. Exactly the picture I was thinking of, and I think that's all you can say, and all that is needed...